Cast size_t to unsigned long.

printf expects unsigned long (%lu) argument, however, size_t doesn't
have to be declared as such. As %zu is C99 (but not C++), it shouldn't
be used directly. Instead, I have to cast value to the correct type.

exec.cpp

@@ -188,7 +188,7 @@ void print_open_fds(void)
     {
         if (open_fds.at(i))
         {
-            fprintf(stderr, "fd %lu\n", i);
+            fprintf(stderr, "fd %lu\n", (unsigned long) i);
         }
     }
 }